Material coatings play an important role in our manufactured products based society. Coatings provide immunity to corrosion, thermal insulation, shielding, as well as appearance enhancement, and an aid in identification.
During the life of many manufactured products, such as bridges, aircraft, automobiles, and ships, painted coatings require removal and replacement for a variety of reasons. For example, refurbishment of the paint on aircraft is a regular maintenance item. Commercial airlines repaint their aircraft about every 4-5 years of service. The United States military typically repaints its aircraft after three years of service, or less. Coatings on the exterior surfaces of large ships or bridges require periodic refurbishment in order to prevent or inhibit corrosion.
The removal of paint from the surfaces of aircraft presents special problems. Such surfaces are large, irregularly shaped, and relatively delicate. Because the surfaces of aircraft are typically lightweight aluminum or organically based composite materials, such surfaces and the underlying substrates are particularly susceptible to damage while undergoing paint removal that could degrade their structural integrity.
Many different methods have been used to remove painted coatings. One type, the "particle medium blast" (PMB) method involves impinging the surface to be stripped with particles such as BB's, plastic media, steel shot, wheat starch, and/or sand. However, PMB methods energetic enough by themselves to remove hardened coatings such as paint may damage delicate surfaces such as found on aircraft and automobiles if they are not carefully managed. For example, if the impinging particles dwell too long at one location, the impinged surface may become pitted or stress hardened. This is especially important with regard to the surfaces of aircraft since pitting or stress hardening may change the loading on that portion of the aircraft. PMB may also damage putty joints often found on aircraft between surface plates.
It is also known in the art to apply chemical compounds to painted surfaces in order to chemically breakdown the layers of paint, thereby stripping the paint away from the surface to be exposed. However, such compounds may pose a risk to human health, are usually toxic, and often not biodegradable. Overall, these types of compounds are difficult and costly to dispose of because they present serious environmental problems.
Mechanical paint removal techniques have also been employed. For example, U.S. Pat. No. 4,836,858, entitled "Ultrasonic Assisted Paint Removal Method" discloses a hand held tool which uses an ultrasonic reciprocating edge placed in contact with the surface to be stripped. Unfortunately, employment of this tool is labor intensive and relies upon the skill of a human operator to use it effectively. Further, control of this tool is a problem when applied to aircraft because the aircraft surface may be damaged if there is excessive tool dwell at one location.
Radiant energy paint removal techniques are also known in the art. One such system uses a laser and video frame grabber in a video controlled paint removal system in which paint is stripped from a surface using the output of the laser to ablate the paint while a video camera converts images of the surface being stripped into electronic data signals. The data signals are used to control the laser output. A processor compares the data signals with parameters stored in a memory to determine whether sufficient paint has been removed from the surface being stripped. If an insufficient amount of paint has been removed, then the surface continues being irradiated by the laser. If the irradiated area has been adequately stripped, the processor directs the laser to ablate another area. A significant problem with the video controlled paint removal system is that the amount of data which is generated and which must be processed is enormous. Hence, real time control of video controlled paint removal systems is extremely difficult.
The use of composite structures manufactured, for example, of fiber reinforced epoxy or other thermoset or thermoplastic composites is becoming increasingly common. Many aircraft and automobiles extensively employ plastic composites for surface structures. Such structures are painted for a variety of reasons including aesthetics, identification, and camouflage. However, such painted surfaces deteriorate under the action of weather and the mechanical forces to which they are subjected, thus requiring removal and replacement.
Other than hand sanding, there are no suitable methods for removing paint from the surfaces of such composites. PMB and mechanical grinding methods sufficiently energetic by themselves to remove paint have proven to damage composite materials. The removal of paint with chemical compounds does not offer a satisfactory solution because such chemicals tend to attack the composites, as well as the paint.
For a variety of reasons, paint removal techniques for removing paint from large surfaces, and surfaces having topological anomalies such as rivets, have not proven wholly satisfactory. Thus, it can be appreciated that coating removal, and particularly, the removal of paint from large and often delicate surfaces such as found on aircraft and automobiles, is a problem that has not been satisfactorily solved.